Fort Okanogan (also spelled Fort Okanagan) was founded as a fur trade outpost by John Jacob Astor’s Pacific Fur Company in 1811. It was built at the confluence of the Okanogan and Columbia Rivers, in what is now Okanogan County, Washington. The fort was the first American-owned settlement in what is now Washington, although its ownership soon passed to the North West Company when the Pacific Fur Company sold out its assets and posts to its rival.

Originally built for the Pacific Fur Company, the North West Company purchased the fort, along with the rest of the Pacific Fur Company, in 1813. In 1821 the North West Company was merged into Hudson's Bay Company, which took over operation of Fort Okanogan as part of its Columbia District. The fort was an important stop on the York Factory Express trade route to London via Hudson Bay.

HBC Governor Sir George Simpson commented about Fort Okanagan during his 1841 visit to the Columbia District:

In 1846, the Oregon Treaty was ratified, ending the Oregon boundary dispute and the joint-occupation of Oregon Country, though the Hudson's Bay Company was allowed to continue use of the fort. However, due to the decline of the transport business in the area, the HBC abandoned the fort in 1860.

The site of the fort was flooded in 1967 by the reservoir Lake Pateros due to the construction of Wells Dam.

Fort Okanogan State Park

Today, Fort Okanogan State Park 48°05′53″N 119°40′42″W﻿ / ﻿48.09806°N 119.67833°W﻿ / 48.09806; -119.67833 overlooks the fort site and the Columbia River. Comprising 45-acre (180,000 m2), the park is for day use and features the Fort Okanogan Interpretive Center, a museum with exhibits about the fort, area pioneers and the fur trapping industry.

The park is located five miles (8 km) north of Brewster, Washington. It is closed during the winter. Admission is free.